fix: migrate to NodeNext ESM for @actions/core v3 compatibility

@actions/core v3 is ESM-only (no CJS bundle, exports-field only).
TypeScript's moduleResolution:node predates exports-field support and
cannot locate the package, breaking both tsc compilation and Jest.

Changes:
- tsconfig.json: module/moduleResolution NodeNext, exclude test files
- package.json: add "type":"module", bump devDeps already from Renovate
- jest.config.js: keep CJS mode for tests via ts-jest inline tsconfig
  override (jest.mock hoisting unreliable in ESM mode with ts-jest v29)
- All source + test files: add .js extensions to relative imports
  (required by NodeNext module resolution)
- git.ts: switch to named import { simpleGit } (default import not
  callable under NodeNext)
- __mocks__/@actions/core.ts: manual mock so Jest bypasses ESM-only
  package; moduleNameMapper redirects all test imports here

All 34 tests pass; tsc+ncc build produces dist/index.js cleanly.

Author: ricogu

.github/actions/core/__mocks__/@actions/core.ts

@@ -0,0 +1,14 @@
+import { jest } from '@jest/globals';
+
+export const getInput = jest.fn();
+export const setOutput = jest.fn();
+export const setFailed = jest.fn();
+export const info = jest.fn();
+export const debug = jest.fn();
+export const error = jest.fn();
+export const warning = jest.fn();
+export const summary = {
+    addRaw: jest.fn().mockReturnThis(),
+    addDetails: jest.fn().mockReturnThis(),
+    write: jest.fn().mockResolvedValue(undefined),
+};